Radiation therapy is one of the most effective tools for cancer treatment. It is well known that the use of proton beams provides the possibility of superior dose conformity to the treatment target as well as providing a better normal tissue sparing, as a result of the Bragg peak effect, compared to photons (e.g., X-rays) and electrons. See, e.g. T. Bortfeld, “An analytical approximation of the Bragg curve for therapeutic proton beams”, Med. Phys., 2024–2033 (1997). While photons show high entrance dose and slow attenuation with depth, protons have a very sharp peak of energy deposition as a function of beam penetration. As a consequence, it is possible for a larger portion of the incident proton energy to be deposited within or very near the 3D tumor volume, thus avoiding radiation-induced injury to surrounding normal tissues that commonly occurs with x-rays and electrons.
Despite the dosimetric superiority characterized by the sharp proton Bragg peak, utilization of proton therapy has lagged behind that of photon therapy. This lag is apparently due to the operating regime (the total operating cost for accelerator maintenance, energy consumption, and technical support) for proton accelerators being at least an order of magnitude higher compared to electron/X-ray medical accelerators. Currently, proton therapy centers utilize cyclotrons and synchrotrons. See, e.g., Y. A. Jongen et al., “Proton therapy system for MGH's NPTC: equipment description and progress report”, Cyclotrons and their Applications, J. C. Cornell (ed) (New Jersey: World Scientific) 606–609 (1996); “Initial equipment commissioning of the North Proton Therapy Center”, Proc. of the 1998 Cyclotron Conference (1998); and F. T. Cole, “Accelerator Considerations in the Design of a Proton Therapy Facility”, Particle Acceleration Corp. Rep (1991). Despite a somewhat limited number of clinical cases from these facilities, treatment records have shown encouraging results particularly for well localized radio-resistant lesions. See, e.g., M. Fuss et al., “Proton radiation therapy (PRT) for pediatric optic pathway gliomas: Comparison with 3D planned conventional photons and a standard photon technique”, Int. J. Radiation Oncology Biol. Phys., 1117–1126 (1999); J. Slater et al., “Conformal proton therapy for prostate carcinoma” Int. J. Radiation Oncology Biol. Phys., 299–304 (1998); W. Shipley et al., “Advanced prostate cancer: the results of a randomized comparative trial of high dose irradiation boosting with conformal protons compared with conventional dose irradiation using photons alone”, Int. J. Radiation Oncology Biol. Phys., 3–12 (1995); and R. N. Kjellberg, “Stereotactic Bragg Peak Proton Radiosurgery for Cerebral Arteriovenous Malformations” Ann Clin. Res., Supp. 47, 17–25 (1986). This situation could be greatly improved by the availability of a compact, flexible, and cost effective proton therapy system, which would enable the widespread use of this superior beam modality and therefore bring significant advances in the management of cancer.
Thus, there remains the problem of providing a practical solution for a compact, flexible and cost-effective proton therapy system. See, e.g., C.-M. Ma et al., “Laser accelerated proton beams for radiation therapy”, Med. Phys., 1236 (2001); and E. Fourkal et al., “Particle in cell simulation of laser-accelerated proton beams for radiation therapy”, Med. Phys., 2788–2798 (2002). Such a proton therapy system will require three technological developments: (1) laser-acceleration of high-energy protons, (2) compact system design for ion selection and beam collimation, and (3) the associated treatment optimization software to utilize laser-accelerated proton beams.
U.S. Patent Application Pub. No. US 2002/0090194 A1 (Tajima) discloses a system and method of accelerating ions in an accelerator to optimize the energy produced by a light source. It is disclosed that several parameters may be controlled in constructing a target used in the accelerator system to adjust performance of the accelerator system.
Simulations of the laser acceleration of protons reported by Fourkal et al., showed that, due to their broad energy spectrum, it is unlikely that laser accelerated protons can be used for therapeutic treatments without prior proton energy selection. If such an energy distribution is achieved, however, it should be possible to provide a homogeneous dose distribution through the so-called Spread Out Bragg's Peak (“SOBP”). Using multiple beams (beamlets) it should also be possible to conform the dose distribution to the target laterally (intensity modulation). Intensity-modulated radiation therapy (“IMRT”) using photon beams could deliver more conformal dose distribution to the target while minimizing the dose to surrounding organs compared to conventional photon treatments. In “On the role of intensity-modulated radiation therapy in radiation oncology”, Med. Phys., 1473–1482 (2002), R. J. Shultz, et al. addressed the role of the intensity-modulated radiation therapy in treatments of specific disease sites. This topic of research is still in its latent stage requiring accumulation and analysis of more data, but the findings of Shultz et al. suggest that at least there could be an advantage of using IMRT for treatments of such sites as the digestive system (colorectal, esophagus, stomach), bladder and kidney.
Giving a homogeneous dose distribution in the target's depth direction may be possible; see, e.g., C. Yeboah et al., “Intensity and energy modulated radiotherapy with proton beams: Variables affecting optimal prostate plan”, Med. Phys., 176–189 (2002); and A. Lomax, “Intensity modulation methods for proton radiotherapy”, Phys. Med. Biol., 185–205 (1999). Accordingly, Energy- and Intensity-Modulated Proton Therapy (“EIMPT”) should further improve target coverage and normal tissue sparing effects. In recent years, the planning and delivery of X-rays has improved considerably so that the gap between the conventional proton techniques and X-ray methods has decreased dramatically. The main pathway of research has been toward the optimization of individual beamlets and the calculation of optimal intensity distributions (for each beamlet) for intensity modulated treatments. See, e.g., E. Pedroni, “Therapy planning system for the SIN-pion therapy facility”, in Treatment Planning for External Beam Therapy with Neutrons, ed. G. Burger, A. Breit and J. J. Broerse (Munich: Urban and Schwarzenberg); and T. Bortfeld et al., “Methods of image reconstruction from projections applied to conformation radiotherapy”, Phys. Med. Biol., 1423–1434 (1990). Unfortunately, the implementation of intensity modulation for proton beams has lagged behind that of photons due to the design limitations of conventional beam delivery methods in proton therapy. See, e.g., M. Moyers “Proton Therapy”, The Modern Technology of Radiation Oncology, ed. J. Van Dyk (Medical Physics Publishing, Madison, 1999). Thus, there remains the problem of providing a combination of a compact proton selection and collimation device and treatment optimization algorithm to make EIMPT possible using laser-accelerated proton beams.
Laser acceleration was first suggested in 1979 for electrons (T. Tajima and J. M. Dawson, “Laser electron accelerator”, Phys. Rev. Lett., 267–270 (1979)), and rapid progress in laser-electron acceleration began in the 1990's after Chirped Pulse Amplification (“CPA”) was invented (D. Strickland, G. Mourou, “Compression of amplified chirped optical pulses,” Opt. Comm., 219–221 (1985)) and convenient high fluence solid-state laser materials such as Ti:sapphire were discovered and developed. The first experiment that has observed protons generated with energy levels much beyond several MeV (58 MeV) is based on the Petawatt Laser at Lawrence Livermore National Laboratory (“LLNL”). See, e.g., M. H. Key et al., “Studies of the Relativistic Electron Source and related Phenomena in Petawatt Laser Matter Interactions”, in First International Conference on Inertial Fusion Sciences and Applications (Bordeaux, France, 1999); and R. A. Snavely et al., “Intense high energy proton beams from Petawatt Laser irradiation of solids”, Phys. Rev. Lett., 2945–2948 (2000). Until then, there had been several experiments that observed protons of energy levels up to 1 or 2 MeV. See, e.g., A. Maksimchuk et al., “Forward Ion acceleration in thin films driven by a high intensity laser”, Phys. Rev. Lett. 4108–4111, (2000). Another experiment at the Rutherford-Appleton Laboratory in the U.K. has been reported recently with proton energy levels of up to 30 MeV. See, e.g., E. L. Clark et al., “Energetic heavy ion and proton generation from ultraintense laser-plasma interactions with solids”, Phys. Rev. Lett., 1654–1657 (2000).
It has long been understood that ion acceleration in laser-produced plasma relates to the hot electrons. See, e.g., S. J. Gitomer et al., “Fast ions and hot electrons in the laser-plasma interaction”, Phys. Fluids, 2679–2686 (1986). A laser pulse interacting with the high density hydrogen-rich material (plastic) ionizes it and subsequently interacts with the created plasma (collection of free electrons and ions). The commonly recognized effect responsible for ion acceleration is a charge separation in the plasma due to high-energy electrons, driven by the laser inside the target (see, e.g., A. Maksimchuk et al., Id., and W. Yu et al., “Electron Acceleration by a Short Relativistic Laser Pulse at the Front of Solid Targets”, Phys Rev. Lett., 570–573(2000)) or/and an inductive electric field as a result of the self-generated magnetic field (see, e.g., Y. Sentoku et al., “Bursts of Superreflected Laser Light from Inhomogeneous Plasmas due to the Generation of Relativistic Solitary Waves”, Phys. Rev. Lett., 3434–3437 (1999)), although a direct laser-ion interaction has been discussed for extremely high laser intensities, on the order of 1022 W/cm2; see, e.g., S. V. Bulanov et al, “Generation of Collimated Beams of Relativistic Ions in Laser-Plasma Interactions”, JETP Letters, 407–411 (2000). These electrons can be accelerated up to multi-MeV energy levels (depending on laser intensity) due to several processes, such as ponderomotive acceleration by propagating laser pulse (W. Yu et al., Id.); resonant absorption in which a part of laser energy goes into creation of a plasma wave which subsequently accelerates electrons (S. C. Wilks and W. L. Kruer, “Absorption of Ultrashort, ultra-intense laser light by solids and overdense plasmas” IEEE J. Quantum Electron., 1954–1968 (1997)); and “vacuum heating” due to the v×B component of the Lorentz force (W. L. Kruer and K. Estabrook, “J×B heating by very intense laser light,” Phys. Fluids, 430–432 (1985)). Because of the number of mechanisms for electron acceleration and the corresponding electric field generation, different regimes of ion acceleration are possible. Understanding the mechanisms of ion acceleration in the interaction of laser pulse with a solid target and quantification of the ion yield in terms of the dependencies on the laser pulse and the plasma parameters are useful for designing laser proton therapy systems.
Having the quantified ion yield of a laser-accelerated proton ion beam alone is typically insufficient for preparing a therapeutically-suitable proton ion dose. Such proton ion beams have a wide energy distribution that further require energy distribution shaping (i.e., the resulting high energy polyenergetic ion beam) to be therapeutically suitable. In addition to needing to shape the polyenergetic beam's energy distribution, beam size, direction and overall intensity need to be controlled to provide proton beams that are therapeutically sufficient for irradiating a target in a patient. Lower-energy protons typically treat shallower regions in a patient's body, whereas higher-energy protons treat deeper regions. Thus, there remains the problem of providing systems and methods for forming therapeutically-suitable polyenergetic ion beams from sources of laser-accelerated high energy protons that are capable of treating a predetermined three dimensional conformal region within a body. Such ion selection systems are presently needed to provide low-cost, compact, ion therapy systems to enable the greater availability of positive ion beam therapy to society.